The present invention relates to a variable valve timing control technology for variably controlling the opening and closing timing of an intake valve and an exhaust valve of an internal combustion engine, and especially to a technology for reducing hydrocarbon emissions at an engine idling condition immediately after an engine start.
There are many unburned hydrocarbons in the exhaust due to the low combustion temperature immediately after an engine start. Further, since the exhaust gas temperature is also low, an exhaust gas purification catalytic converter is not activated. Therefore, hydrocarbon emissions are likely to increase. Especially, at the end period of exhaust stroke, unburned hydrocarbons adhered to an inner wall of a cylinder are peeled off to be discharged, which promotes an increase of a hydrocarbon emission amount from the cylinder.
Therefore, in Japanese Unexamined Patent Publication No. 2000-8896, a variable valve mechanism is disposed at an intake valve, wherein, at an engine cold period, the opening time of the intake valve is controlled to be advanced so that overlap amounts of the intake valve and an exhaust valve are increased. Thereby, hydrocarbons with a small molecular weight generated in the vicinity of a top dead center are sucked back to an intake system to increase a hydrocarbon amount remaining in the cylinder to be re-burned at the next stroke, thereby achieving the reduction of the unburned hydrocarbon emissions.
However, at the time immediately after the engine start, if the valve overlap amounts are increased by advancing the opening time of the intake valve as above, at a first cycle more unburned hydrocarbons can be remained as an inner EGR, while there is a possibility that the combustion becomes unstable at a second cycle or thereafter, leading to the deterioration of drivability.
Further, hydrocarbons with a relatively large molecular weight are increased accompanied with this unstable combustion. In order to eliminate these hydrocarbons with a relatively large molecular weight, there is a need to dispose a HC adsorbent using zeolite and the like in an exhaust passage, in addition to a three-way catalytic converter.
In view of the foregoing conventional problems, the present invention has been achieved and an object of the invention is to reduce hydrocarbon emissions from an engine immediately after an engine start without the deterioration of engine drivability, by optimally controlling the opening and closing timing of an intake valve and an exhaust valve.
In order to achieve the above object, with the present invention, the valve timing of an intake valve and an exhaust valve is controlled as follows.
An engine load and an engine rotation speed are detected, and then the reference valve timing of the intake valve and the exhaust valve is set based on the detected engine load and rotation speed.
An engine start and an engine idling condition are detected. At the engine start time and in the engine idling condition during a predetermined period of time after the engine start, the valve timing of the intake valve and the exhaust valve is controlled, while maintaining the valve overlap amounts to be substantially the same, so that a valve overlap center is positioned at an advance side from an exhaust top dead center with respect to the reference valve timing.
According to this constitution, without an increase of an inner EGR amount (burned gas amount), unburned hydrocarbons included in the inner EGR can be increased and more unburned hydrocarbons can be remained in a cylinder to be re-burned in a next stroke. Accordingly, hydrocarbon emissions can be reduced without damaging the combustion performance.
Further, in the engine idling condition after the above predetermine time has elapsed, the valve timing of the intake valve and the exhaust valve is controlled to a retard side with respect to the reference valve timing, while maintaining the valve overlap amounts to be substantially the same.
According to this constitution, it becomes possible to make the unburned hydrocarbons to remain for a long period in the cylinder, the temperature of which has risen due to the elapse of the predetermined period, so that oxidization of the unburned hydrocarbons after combustion is promoted. As a result, the hydrocarbon emissions from the engine can be reduced effectively.
Further, in a case where the predetermined period of time is set based on the detected engine temperature, this predetermined period of time may be set based on an elapsed time after a measured engine start or a cycle number after the measured engine start.
Moreover, switching from an advance control to a retard control with respect to the above reference valve timing is carried out gradually.
According to this constitution, a change of the valve timing can be made without damaging the engine drivability.
The other objects and features of this invention will become understood from the following description with accompanying drawings.